The present invention relates to an improved speaker system and audio system which allow a low-pitched sound to be output with a flat characteristic.
In the field of speakers including a speaker unit housed in a cabinet (commonly called “speaker systems”), there have been proposed various internal structures (so-called “enclosures”) of the cabinets which, in order to enhance a low-pitched-sound reproduction efficiency, can not only lower a low-band resonance frequency f0 of the speaker unit itself but also allow low-pitched sounds to be output in an accentuated fashion. One example of such internal structures of the speaker cabinets is proposed in “Encyclopedia of Speaker & Enclosure” published in 1999 by Seibundo Shinko Sha under editorial supervision by Saeki Tamon.
FIGS. 1A-1C are views showing examples of interior structures of speaker cabinets. Specifically, FIG. 1A shows the interior structure of a bass-reflex type cabinet, which has a cylindrical opening (bass reflex port) in the same plane as the speaker. This interior structure permits accentuated output of a low-pitched sound, taking advantage of the phenomenon that air within the cabinet and air within the bass reflex port resonate with each other at a particular frequency through the Helmholtz resonance. The resonance frequency fr can be expressed byfr=√{square root over ( )}(S/P)/2π, where S represents an inner cubic capacity of the cabinet and P represents an inner cubic capacity of the port.
Low-frequency reproduction characteristic of the speaker can be improved by designing the resonant frequency fr to be slightly lower than the low-band resonance frequency f0 of the speaker unit.
Further, FIG. 1B is a view showing the interior structure of a “quarter tube” cabinet. The quarter tube serves to accentuate a low-pitched sound through tube resonance, and a resonance tube (closed tube) having a length h is formed by a partition plate provided on the rear surface of the speaker. Lowest resonance frequency Fr can be expressed by Fr=M/4h (where M represents the sound speed). Further, FIG. 1C a view showing the interior structure of a “transmission line” cabinet that takes advantage of the tube resonance and the Helmholtz resonance.
With each of the aforementioned cabinet structures, it is possible to accentuate low-pitched sounds through resonance; however, because only one low-band resonance frequency is permitted in the speaker unit and enclosure, the resonance action can be utilized only in a very narrow frequency band range. Thus, the bass-reflex type cabinet can present a characteristic as illustrated in FIG. 2A. Particularly, where the cabinet is designed to achieve strong resonance, however, balance in frequency characteristic between the speaker unit and the bass reflex port would be lost as seen in FIG. 2B, which would often make it difficult to obtain a flat characteristic over a wide low frequency band range.